Final answer:
La Niña causes the jet stream to shift further south over North America, leading to distinct weather pattern changes across the continent.
Step-by-step explanation:
The weather variation that shifts the jet stream further south over the North American continent is La Niña. La Niña represents the cool phase of the El Niño Southern Oscillation (ENSO), characterized by below-average sea surface temperatures (SST) across the central and eastern Pacific Ocean. During a La Niña event, the prevailing easterly trade winds strengthen, which can push the jet stream further south, affecting weather patterns across North America. This typically results in cooler, wetter weather in the Pacific Northwest and drier, warmer conditions in the southern portion of the United States.
Contrastingly, El Niño is the warm phase of ENSO and is associated with the opposite effects, such as a northward shift of the jet stream which can lead to warmer, drier conditions in the north and wetter conditions in the south.